Fiber-reinforced composite materials (hereinafter referred to as FRPs) reinforced with reinforcing fibers have characteristics such as superiority in specific strength, specific rigidity, and the like, as well as being light in weight. By making use of such characteristics, fiber-reinforced composite materials have been widely used for aircraft structural materials, automotive parts, and sport applications such as rackets and golf shafts.
In a general method for molding an FRP, an intermediate material, called a prepreg, obtained by impregnating a reinforcing fiber with a thermosetting resin is used, and the prepreg is cured and molded by autoclave molding, vacuum bag molding, press molding, or the like. A resin for the prepreg needs to have both stability at normal temperature and curability upon application of heat or the like. In general, thermosetting resins such as epoxy resin compositions are often used as the resin.
In addition, methods of molding an FRP without using a prepreg are known. General molding methods without using a prepreg include resin transfer molding (hereinafter sometimes referred to as RTM), vacuum RTM molding, RFI molding, filament winding molding, pultrusion molding, and the like. In these moldings, thermosetting resins having low viscosities such as glycidyl ether-type epoxy, alicyclic epoxy, vinyl ester, and unsaturated polyester resins are used as the resin.
Here, heating for a certain time is necessary for curing a thermosetting resin, and a long time is required for the molding, when the time to raise the temperature to a predetermined temperature, and the time to cool the molded article to a temperature low enough for the molded article to be taken out are also taken into consideration. Hence, there is a limitation in reduction of costs by improving the production cycle. In addition, there is a problem of high energy costs because of continuous heating.
In this respect, molding methods using an ultraviolet curable resin have been developed in recent years as molding methods conducted in a short time without heating. As the ultraviolet curable resin, a radically polymerizable resin such as an acrylic-based resin or a vinyl ester-based resin is used. The radically polymerizable resin is curable upon irradiation with ultraviolet rays in a short time, without heating. Hence, the radically polymerizable resin allows molding in a shorter time and with less energy than those of thermal curing, so that productivity of molding can be remarkably improved. Besides, radically polymerizable resins have a wide variety, including a variety of polymerization initiators, and are readily available at relatively low costs.
However, radically polymerizable acrylic-based resins and vinyl ester-based resins of the ultraviolet curable type are inferior to epoxy-based resins in terms of toughness, elongation, and ease of handling. For this reason, an epoxy resin has been sought which can be cured in a short time with heat or ultraviolet rays, and which makes it possible to efficiently obtain a cured product having superior mechanical strength.
Patent Document 1 describes use of a cationically polymerizable epoxy compound curable with active energy rays as a resin contained in a prepreg.
However, Patent Document 1 fails to specifically describe curing of the cationically polymerizable compound with active energy rays, but employs conventional autoclave molding. Accordingly, molding cost reduction is not achieved. In addition, the resins described in Patent Document 1 are not suitable for RTM molding, because these resins are resins for prepregs, and hence have high viscosity.
Moreover, it is known that bisphenol A-type epoxy resins having a flexible skeleton such as an oxyalkylene group are used in RTM molding. However, cured products of conventional modified bisphenol A-type epoxy resins do not have sufficient mechanical strength.
Patent Document 2 describes an RTM molding method which enables curing with heat or ultraviolet rays in a short time, and efficient molding of a cured product having superior mechanical strength. The molding method is characterized by the use of a resin composition described in Patent Document 3, which is referred to as a CCP (chain curing-type resin composition). The resin composition is such that when heat or ultraviolet energy is applied to the resin composition, another kind of first energy different from the energy from an energy source is autogenously generated within the resin, and the same kind of second energy is successively generated by the generated energy, so that the resin composition can be chain cured by means of the first and second energies, or both the first and second energies and the energy from the energy source.
However, the resin composition described in Patent Document 3 has a problem that a cured product of the resin composition is poor in mechanical strength.